Modification of naturally occurring polypeptides which have therapeutic value is often attempted in an effort to increase their biological activity. Several methods have been employed to increase the biological activity of therapeutic proteins. These methods often focus on increasing the size of the therapeutic agents. For example, the size of a protein can be increased through chemical conjugation with a reagent such as polyethylene glycol (PEG) (Knusli, C. et al., Brit. J. Haematol. 82:654-663 (1992)). This procedure, also known as "PEGylation", has been reported with several protein agents, first as a means to reduce antigenicity, but also as a way to increase biological activity.
Another method of increasing a protein's size is through chemical cross-linking with another protein. For example, to increase the antigenicity of a protein, chemical cross-linking agents are used to conjugate the immunogenic protein to a carrier molecule such as immunoglobulin or serum albumin.
However, the conjugation of chemical compounds or inert molecules to a polypeptide often results in a significant decrease of the overall biological activity, are of selected biological activity of the polypeptide, (Knusli, C., et al., Brit. J. Haematol., 82:654-663 (1992)). These conjugations must be designed such that the resulting modified polypeptide remains therapeutically efficacious and retains the desired biological properties of the unmodified, wild type (i.e., naturally-occurring) polypeptide (Satake, R., et al., Biochem. Biophys. Acta. 1038:125-129 (1990)).
Erythropoietin (EPO) is a glycoprotein hormone involved with the growth and development of mature red blood cells from erythrocyte precursor cells. It is a 166 amino acid polypeptide that exists naturally as a monomer. (Lin, F- K., et al. Proc. Natl. Acad. Sci. USA 82:7580-7584 (1985)).
Several forms of anemia, including those associated with renal failure, HIV infection, blood loss and chronic disease can be treated with this hematopoietic growth factor. Erythropoietin is typically administered by intravenous or subcutaneous injection three times weekly at a dose of approximately 25-100 U/kg. Though quite effective, this form of therapy is very expensive. Estimates for the treatment of chronic dialysis patients have ranged from $8,000-10,000 per patient per year.
Another problem encountered in the practice of medicine when using injectable pharmaceuticals is the frequency at which those injections must be made in order to maintain a therapeutic level of the compound in the circulation. For example, erythropoietin has a relatively short plasma half-life (Spivak, J. L., and Hogans, B. B., Blood, 73:90 (1989); McMahon, F. G., et al., Blood, 76:1718(1990) ) , therefore, therapeutic plasma levels are rapidly lost, and repeated intravenous administrations must be made. An alternative route of administration is subcutaneous injection. This route offers slower absorption from the site of administration, thus causing a sustained release effect. However, significantly lower plasma levels are achieved and, thus, a similar frequency of injection, as is required with intravenous administration, must be used to get a comparable therapeutic effect.